Most viruses...
A) are rarely found in the natural world
B) switch between pathogenic and non-pathogenic modes of existence
C) can use host machinery to produce more of themselves
D) are persistent and innocuous within host cells
E) kill the cell they infect

C) can use host machinery to produce more of themselves

Explanation:

Viruses are organisms that are incapable of replicating on their own, hence, they need to infect a living cell (host) in order to reproduce. This process of infection is regarded as their reproductive life cycle. They make use of the host cell's ability to transcribe, translate and replicate to multiply themselves.

Most viruses infect an host cell by initially attaching to certain receptor site on the host's membrane, after which they incorporate their genetic material into the host cell's DNA. Some viruses kill their host after replicating (lytic cycle) or keep them alive in order to persistently replicate their virions (lysogenic cycle).